Purification and reclamation of moderator-coolants



April 2, 1968 J. F. BLACK 3,376,357

PURIFICATION AND RECLAMATION OF MODERATOR-COOLANTS Filed Jan. 27. 1966STILL NUCLEAR REACTOR -'1-2| 26 I2 mxme TANK H I seams-use 24L.- l3

\PARAFFINIC PRECIPITANT STORAGE TANK JAMES; F. BLACK mvE moR av M PATENTATTORNEY United States Patent 3,376,357 PURIFICATION AND RECLAMATION 0FMODERATOR-COOLANTS James F. Black, Convent Station, N.J., assignor, bymesne assignments, to the United States of America as represented by theUnited States Atomic Energy Commission Continuation-impart ofapplication Ser. No. 124,189,

July 14, 1961. This application Jan. 27, 1966, Ser.

5 Claims. (Cl. 260-674) This application is a continuation-in-part ofapplication Ser. No. 124,189, filed July 14, 1961, now abandoned.

This invention relates to removing contaminants from nuclear reactorcoolants or moderator-coolants. In particular, the invention relates toa process for causing undesirable high boiling polymer impurities inorganic coolants to coagulate so that they may be easily removedtherefrom.

Moderators are materials which are placed in a nuclear reactor to slowdown newly born neutrons from their original high velocities. A coolantis generally a fluid substance which is used for cooling any part of areactor in which heat is generated. It can also be used as a heatexchange medium. As used herein the term coolant includes coolants perse, as vvell as moderator coolants.

The feasibility of using hydrocarbon fluids as coolants for nuclearreactors has been demonstrated in the Organic Moderator ReactorExperiment (OMRE) currently being operated by Atomics International forthe US. Atomic Energy Commission. The coolant used in this reactor is asynthesized mixture of polyphenyls comprising primarily isomers ofterphenyls. On prolonged service within a nuclear reactor, degradationproducts, such as highboiling polymers are formed within the moderatorcoolant as the result of chemical reactions induced by radiation and bythe heat liberated during fission. A buildup of these degradationproducts within a reactor can cause surface fouling of the fuelelements. Such fouling can cause a local temperature rise which canaccelerate the decomposition rate of a coolant which, in turn, canultimately cause damage to the reactor. Severe damage could ultimatelyresult in a shutdown of the reactor. Conventionally, these high-boilingpolymers, known as high boilers, and other degradation products areperiodically removed from the coolant by distillation and fresh coolantis added as makeup.

This invention resides in the discovery that the high boilers present inorganic coolants as degradation products can be easily and convenientlyremoved by a process of causing these impurities to coagulate and thusprecip-.

itate out from within the main body of the coolant. The

maintained in a liquid phase under the purification condi tions ofelevated temperature and pressure.

The process of the invention may be readily understood by the attacheddrawing illustrating a preferred embodiment thereof.

Referring now to the attached drawing, coolant from storage tank 10 isflowed through line 25 into nuclear reactor 24, wherein decomposition ofaromatic hydrocarbons occurs by radiolysis and pyrolysis causing formation of high boilers which become colloidally dispersed in the hotliquid coolant. To prevent excessive buildup of these high boilers, aside stream of used coolant to be purified is taken off from the nuclearreactor 24. through line 26, through valve 12, then through line 13 tomixing tank 14. Paraflinic precipitant from storage tank 15 is flowedthrough line 16 to mixing tank 14. The mixing of the used coolant withthe precipitant causes the high boiler colloidal impurities to coagulateinto agglomerates which tend to precipitate. The resulting slurry ofsaid agglomerates in the liquid coolant is removed through line 17 fromthe mixing tank 14 to a centrifuge or separator 18. Solid agglomerateseparated from the liquid coolant is removed from separator 18 throughline 19. The remaining mixture of precipitant and liquid coolant isflowed through line 20 to still 21 where precipitant is flashed off andreturned to storage tank 15 through line 22 for reuse in the process.Used coolant is returned to storage tank 10 by line 23. When desired,coolant containing impurities can be flowed from the bottom of storagetank 10 through line 11 to the three-way valve 12, thence by line 13 tomixing tank 14 for repurification by mixing therein with precipitantwhich coagulates high boilers.

The manner in which the deleterious polymeric high boilers are formed inaromatic hydrocarbon coolants, including the fused ring type aromaticsand terphenyl type, by radiolytic and thermal decomposition is describedin Atomic Energy Commission reports NAA-SR- 6920 entitled CoolantDecomposition Rates and IDO- 16907 entitled Low Cost OrganicModerator-Coolants for Nuclear Reactors.

The high boilers which are the potential deposit formers in the aromaticcoolant stream flowing through a nuclear reactor are present as acolloidal suspension of solid polymers that have high melting points. Toa large extent these polymers are dispersed in benzene at temperaturesabove the normal boiling temperature of benzene maintained in liquidphase under pressure. The liquid paraffin hydrocarbons definitely act ascoagulants of the high-boiling polymers having molecular weights above230 as shown by data on precipitation of these polymers from benzenesolution.

TABLE I.PRECIPITATION OF HIGH BOILERS FROM BEN- ZENE SOLUTION TERPHENYLMIXTURE CONTAINING HIGH BOILERS Percent Hexane Percent Higher BoilersAverage Molecular Precipitant Added to Precipitated Weight ofPrecipitated Solution High Boilers In the experiments giving theforegoing data the precipitates were black solids. As more precipitantwas added, more of the high boilers were precipitated.

The paraflin hydrocarbon acts as a coagulant rather than as a solvent ordiluent as shown by the fact that under the coagulating conditions asample of the high boilers in the amount of 25 g. and having an averagemolecular weight of 600 was dissolved in 1 cc. of benzene whereas only1.69 g. of another 25 g. portion of said high boilers was dissolved in 1cc. of hexane. The solute in the hexane was found to have a loweraverage molecular weight of 500.

The coagulating temperature can be controlled to control the amount ofprecipitation and molecular weight of the high boilers precipitated;therefore the temperature is raised to at least 100 F. but below thecritical temperature of the parafiinic precipitant and the pressure ismaintained high enough to prevent bubbling of the precipitant withoutsubstantial effect on the precipitation.

The high boilers have a complex composition of polycyclic aromatics,i.e., mostly above 5 cyclic rings per molecule, but they are differentfrom straight-run or cracked asphalts in not containing resins and oilswhich have sulfur, nitrogen, and oxygen constituents. The asphalts haveconsistencies of sticky amorphous to plastic masses. The high boilerswhich are to be separated from the aromatic coolants are unlike asphaltsin that they can be separated as powdery solids if they are preventedfrom being oxidized. Oxidation of these high boilers tends to occur in avacuum distillation due to leakage of air into the still, and theoxidized high boilers then give handling difiiculties, e.g., formationof tarry residue in the fuel elements.

The process of the invention is also applicable to those coolants whichare prepared from an aromatic extract of a gas oil boiling rangecatalytic cycle stock which may be subjected to pyrolysis and/orvisbreaking to improve its stability in a nuclear reactor.

The preferred precipitants used to cause the impurities to coagulate areC C paraffinic hydrocarbons. Hexane and heptane are particularlypreferred. As specific examples of such materials which can be used,there are butane, pentane, hexane, heptane, octane, nonanc, decane,dodecane and mixtures thereof. The preferred amount of precipitant to beemployed at the deasphalting step is between about 2-10 volumes ofprecipitant per volume of coolant charged to the coagulating step. Thetemperature at which the coagulating step is conducted can be within therange of about 80 to 250 F. Preferably the mixture of precipitant withthe aromatic hydrocarbon coolant i carried out under pressure, e.g., 2to 20 atmospheresto maintain the paraffinic hydrocarbons in liquid stateuntil the precipitate is formed and removed. Moreover, operating undersuch pressure enables the paraffinic precipitant to be easily flashedoff at lowered pressure, e.g., at atmospheric pressure when separationfrom the coolant is desired.

The aromatic extract of catalytic cycle stock is particularly preferredas the coolant to be employed in the process of the invention. However,other suitable materials such as the various petroleum streams disclosedin Atomic Energy Commission report TID 6367 entitled Petroleum RefineryStreams as Prospective Reactor Coolants: Thermal StabilityInvestigations, can be used. As used herein, catalytic cycle stockextracts are obtained from distillates boiling in the gas oil boilingrange which are subjected to conventional catalytic cracking. Catalyticcycle stock is that portion of the gas oil which is uncracked. Thecatalytic cycle stock is then extracted by conventional solventextraction processes such as phenol extraction, sulfur dioxide,furfural, nitrobenzene, and the like. The solvent extraction separates aparafiinic fraction and a highly aromatic extract. It is the highlyaromatic extract which becomes subjected to radiolysis and pyrolysis inthe nuclear reactor. The aromatic extract fraction of the catalyticcycle stock preferably boils within the range of about 500 F. to about900 F. and comprises principally aromatic compounds. It is preferred,but not essential, that these aromatic extracts of catalytic cyclestocks be subjected to conventional desulfurization, such ashydrofining, to reduce the sulfur content of the extract, and/or ahydrodealkylation to lower the alkyl side chain content. Also, othermaterials such as terphcnyls and the like can be used in the invention.

The suitable aromatic hydrocarbons obtainable from aromatic extracts ofpetroleum fractions produced by catalytic cracking when sufiicientlyrefined are low in cost relative to terphenyl oils, which they resemblein boiling range, density, viscosity, tendency to solidify at moderatetemperatures, and stability both radiolytic and thermal. They do notdecompose appreciably at temperatures up to 750 F. They are mostly fusedaromatic ring compounds that have no or few alkyl side chains, likephenanthrene, pyrene, fluorene, which have melting points substantiallyabove 100 F., boiling and sublimation points above 500 F. under 1 atm.or below 1 atm. pressure, and molecular weights averaging from 180 to260. When they undergo radiolytic and thermal decomposition attemperatures up to 750 F. for a period of 1 hour they generate a smallamount of gas, and high boiling polymers which have higher molecularweights and higher melting points, and lower boiling hydrocarbons whichare normally liquid and in the volatility range of benzene to diphenyl.

The low boiling hydrocarbons, -i.e., with boiling points 176 F. to 500F., become present typically in amounts of about 0.5 to 15 wt. percentof the total coolant in the nuclear reactor while the high boilers aremaintained at a concentration below about 30 wt. percent in thecirculating coolant at reactor operating temperatures of 500 to 750 F.and pressures ranging from 2 to 20 atms., with venting of gas and lowboilers from the reactor and purifying a side stream of the circulatingcoolant to remove high boilers.

Suitable organic coolants for use in the present invention also includehydrocarbon fractions boiling within the range of about 550 to 800 F.Preferably this fraction should have a boiling range of at least 100 F.,a pour point of less than about F. and preferably containing not morethan about 5 wt. percent of extraneous materials having neutron crosssections of more than 10 barns. These coolants preferably haveviscosities of from 32 to 2,000 SUS, e.g., 50 to 1,000 SUS at F.Suitable examples of coolant materials of this type include petroleumaromatic hydrocarbon fractions boiling in the gas oil range and higher,low sulfur content virgin gas oil fractions, desulfurized gas oilfractions of crude oils, gas oil boiling range cycle stocks obtainableby the catalytic cracking of petroleum hydrocarbons, aromatic extractsof such catalytic cycle stocks, heating oil boiling range fractions, andthe like. These liquid fractions contain polycyclic aromatichydrocarbons which become solid at temperatures above 80 F. andtherefore should not be cooled much below 80 F. in subjecting thecoolant to the treatment for precipitating out agglomerates.

Additives may also be incorporated in the improved coolants of theinvention to beneficially influence their behavior either in storage orin service. Thus, materials can be added to improve radiation,oxidation, or corrosion resistance, to prevent the deposition of sludgeor coke on fuel elements and heating exchanger surfaces, and to improveviscosity temperature characteristics.

With respect to such additive materials, it is an additional feature ofthe invention that conventional detergent type materials can beincorporated in the circulating coolant to reduce the rate at whichdeposits are laid down on the fuel element surfaces. This will permitthe reactor to be operated at higher fuel element surface temperatures.Thus, greater steam cycle efliciency and high power density from thereactor core will result. The detergents to be used for such a purposeshould be preferably nonionic detergents. Nonionic detergents will notbe affected by the high ionization density due to the intense field ofionizing radiation Within the core. If ionic detergents are employed,they are preferably of the type which will place a negative charge onthe suspended high boilers in the coolant. Suitable nonionic detergentsinclude polyethylene glycol alkylphenyl ethers, alkyl phenoxypolyoxyethylene Example I One hundred g. of a cycle stock extract werepyrolyzed for 24 hours at a temperature of 750 F. 600 g. of heptane werethen added to the pyrolyzed cycle stock extract. Twenty-one andeight-tenths g. of insoluble red powder were formed by the heptaneaddition and separated by filtering the cycle stock/heptane mixture offunder vacuum through a paper filter using a Buckner funnel. The filtratewas then distilled off at atmospheric pressure leaving 62.6 g. of apyrolyzed cycle stock extract. This extract was further distilled undervacuum for purposes of analysis and the following breakdown over thedistillation range was obtained.

G. Light ends 0.7 140 C. 0.2 mm. mercury of pressure 9.4 145-197 C. at0.2 mm. mercury of pressure 28.3 197-240 C. at 0.2 mm. mercury pressure12.6 Residue 8.3

Example H One hundred g. of the identical pyrolyzed cycle stockextraction of Example I were subjected to distillation under thefollowing conditions with the following results:

The above data show that the process of the invention separatesimpurities from a coolant equally as elfectively as vacuum distillationprocedures. The trend is for operating temperatures of nuclear reactorsto gradually increase. As these temperatures increase it becomesincreasingly difiicult for a'vacuum distillation procedure toetfectively remove high-boiling constituents economically. The use ofthe technique of the invention obviates any need for distillation at lowpressures. Thus, expensive and cumbersome apparatus is eliminated infavor of a system which is operated with simple, easily obtainable andeasily constructed apparatus.

The aromatic extract of a catalytic cycle stock used above had thefollowing specification.

TABLE II.AROMATIC HYDROCARBON COOLANI Aromatic extract of Property: cat.cycle stock Gravity, API 0.2 Viscosity, SSU:

At 100 F 423 At 130 F 143 At 210 F"- 44 Conradson carbon, wt. percent1.70 Pour point, F Aniline point, F 72 Average boiling point, F 745 ASTMdistillation, F.:

olf at 681 50% off at 736 90% oil at 827 Composition: wt. percent:

Paraflins 3.0 Noncondensed naphthenes 2.6 Condensed naphthenes 5.7Aromatics 73.0 Aromatic sulfur compounds 11.7 Nonhydrocarbon fraction4.0

Aromatic extract of cat. cycle stock Elementary analysis-percent byweight:

Nitrogen 2.41 Sulfur Nil Silicon Nil Sodium Nil Magnesium Nil CalciumNil Aluminum Nil Vanadium Nil Chromium Nil Iron Nil Nickel Nil Thecoolant containing mainly terphenyls, polynuclear aromatic hydrocarbonshaving fused or condensed rings as in phenanthrene, pyrene, and fiuoreneis maintained at temperatures above F. in being purified andrecirculated to the zone where the coolant is subjected to nuclearradiation and heat in the nuclear reactor to prevent solidification ofthese aromatic hydrocarbons. The coagulation is effected with propane,propane mixed with ethane, or butane at temperatures in the range of 100to 200 F. and can be effected at temperatures of 200 up to 500 F. withthe higher paratfins used as precipitants. The paraffins can be flashedoff at temperatures in the range of 100 to 500 F. even undersuperatmospheric pressures.

Various known devices can be used, such as pumps, heat exchangers,valves, and meters for controlling the flow, temperature, and pressureof the coolant circulated to and from the nuclear reactor. This work wasperformed under the terms of Contract No. MA-1814 between Esso Researchand Engineering Company and the Maritime Administration.

What is claimed is:

1. A method of removing high-boiling polymers having molecular weightsabove 300' from a nuclear reactor coolant which is an aromatichydrocarbon liquid that boils principally within the range of about 550to 800 F. and contains polynuclear aromatic hydrocarbons having averagemolecular weights of 180 to 260, said polymers having been formed byradiolysis and pyrolysis of hydrocarbons in said liquid and beingcolloidally dispersed in said liquid, which comprises contacting saidliquid containing the polymers with a C to C paraflinic hydr0- carbonliquid admixed as a precipitant in an amount to cause said polymers tocoagulate into solid agglomerates at temperatures above 100 F. under apressure which maintains the paraflinic hydrocarbon in liquid phase,separating said solid agglomerates of the polymers from the resultingliquid mixture of aromatic hydrocarbons and precipitant at above 100 F.,distilling of parafi'inic precipitant from said aromatic hydrocarbonliquid which remains unvaporized to recover aromatic liquid freed of thehigh-boiling polymers for reuse as nuclear reactor coolant.

2. The method as defined in claim 1, wherein said highboiling polymersare formed at temperatures of about 500 to 750 F. in the aromatichydrocarbon liquid coolant under pressures of 2 to 20 atmospheres, andin which the paraffinic hydrocarbon precipitant is easily flashed offunder lowered superatmospheric pressure at temperatures between 100 F.and 500 F. from the aromatic hydrocarbon liquid after said separation ofcoagulated high-boiling polymers therefrom.

3. The method as defined in claim 1, whereinthe aromatic hydrocarbonliquid used as the nuclear reactor coolant contains mainly terphenyls asthe polynuclear aromatic hydrocarbons.

4. The method as defined in claim 1, wherein the aromatic hydrocarbonliquid used as the nuclear reactor 4 coolant contains mainly polynucleararomatic hydrocar bons having fused rings as in phenanthrene, pyrene,arid fiuorene.

5. The method as defined in claim 1, wherein a side stream of thearomatic liquid coolant used in a zone Where nuclear radiation and heatforms the high-boiling polymers is withdrawn from said zone for removingthe polymers and is maintained at temperatures above 100 F. while saidpolymers are coagulated and separated and until the liquid coolant isreturned to said zone to prevent solidification of polynuclear aromatichydrocarbons having average molecular Weights of 180 to 260 that areprincipal components of the coolant.

No references cited.

DELBERT E. GANTZ, Primary Examiner.

C. SPRESSER, Assistant Examiner.

1. A METHOD OF REMOVING HIGH-BOILING POLYMERS HAVING MOLECULAR WEIGHTSABOUT 300 FROM A NUCLEAR REACTOR COOLANT WHICH IS AN AROMATICHYDROCARBON LIQUID THAT BOILS PRINCIPALLY WITHIN THE RANGE OF ABOUT 550*TO 800*F. AND CONTAINS POLYNUCLEAR AROMATIC HYDROCARBONS HAVING AVERAGEMOLECULAR WEIGHTS OF 180 TO 260, SAID POLYMERS HAVING BEEN FORMED BYRADIOLYSIS AND PYROLYSIS OF HYDROCARBONS IN SAID LIQUID AND BEINGCOLLOIDALLY DISPERSED IN SAID LIQUID, WHICH COMPRISES CONTACTING SAIDLIQUID CONTAINING THE POLYMERS WITH A C3 TO C12 PARAFFINIC HYDROCARBONLIQUID ADMIXED AS A PRECIPITANT IN AN AMOUNT TO CAUSE SAID POLYMERS TOCOAGULATE INTO SOLID AGGLOMERATES AT TEMPERATURES ABOVE 100*F. UNDER APRESSURE WHICH MAINTAINS THE PARAFFINIC HYDROCARBON IN LIQUID PHASE,SEPARATING SAID SOLID AGGLOMERATES OF THE POLYMERS FROM THE RESULTINGLIQUID MIXTURE OF AROMATIC HYDROCARBONS AND PRECIPITANT AT ABOVE 100*F.,DISTILLING OF PARAFFINIC PRECIPITANT FROM SAID AROMATIC HYDROCARBONLIQUID WHICH REMAINS UNVAPORIZED TO RECOVER AROMATIC LIQUID FREED OF THEHIGH-BOILING POLYMERS FOR REUSE AS NUCLEAR REACTOR COOLANT.